Plastics molding is a well-known art. Plastics are typically resin, or resin-based compositions, capable of taking and holding permanent shapes when subjected to interaction with a molding apparatus. Thermoplastic resins, such as polypropylene, are one general type of plastic. A thermoplastic resin is first heated beyond its melting point in a barrel and screw mixer of a molding machine. The melted resin is then forced into an injection molding cavity. Once in the cavity, the melted resin conforms to the shape of the cavity. The cavity is then cooled to remove heat energy from the resin until it solidifies by polymeric crystallization. Once the resin solidifies, the molding cavity is opened and the completed article is removed. Thermosetting resins, such as, epoxy resin, are another general type of plastic. Thermosetting resins rely on a chemical reaction, rather than temperature, to change from liquid to solid. In an epoxy resin, two types of liquid materials are mixed and then injected into a molding cavity. The liquid epoxy resin conforms to the shape of the cavity. A chemical reaction between the two materials causes the epoxy resin to become a solid material.
Referring now to FIGS. 11A through 11D, a prior art molding apparatus is shown in cross sectional view to illustrate important aspects of a typical molding process. Referring particularly to FIG. 11A, an injection molding apparatus 300 includes a molding die of A side 304 and B side 308. The A side 304 and B side 308 are held in a press, not shown. The press is capable of forcing and holding the A side 304 and B side 308 together under pressure and of opening the molding die by forcing the A side 304 and B side 308 apart, as shown in FIG. 11B. Referring again to FIG. 11A, the press mechanically couples to the sides 304 and 308 and the die structure 212 to perform operations requiring movement of the components and application of pressure. In addition, the press conducts incoming liquid resin material and coolant to molding die sides 304 and 308 and outgoing air from the die during cavity fill. The molding apparatus 300 also includes ejector pins 320, retainer plate 324, and an ejector plate 328. The operation of these components is described below.
In the closed position, shown in FIG. 11A, the inner surfaces of the A-side 304 and B-side 308 create a mold cavity 316. During a molding cycle, liquid resin 318 is prepared by mixing and melting (if a thermoplastic) in an injection barrel, not shown. The liquid resin 318 is then injected from the barrel, through a sprue, not shown, on either the A or B sides 304 and 308. The liquid resin flows through a series of small runners, not shown, machined into the faces of the A and B sides 304 and 308. The liquid resin 318 then enters the mold cavity 316 through gates, not shown. The amount of liquid resin 318 required to fill the sprue, runners, and the molding cavity 316 is typically called a shot. As the shot of liquid resin 318 enters the mold cavity 316, air trapped in the cavity 316 escapes through small vents, not shown, ground into the parting line of the mold sides 304 and 308.
Once the mold cavity 316 has completely filled, the liquid resin 318 entirely replicates the shape of the cavity 316. This liquid resin 318 is forced into the mold cavity 316 under pressure to ensure that there are no voids. To maintain the liquid resin 318 under pressure, the sides 304 and 308 are held together under pressure. If thermoplastic resin is used, then the liquid resin 318 must be cooled to complete solidification. Cooling is typically performed by passing a coolant, such as water, through pathways, not shown, milled through the sides 304 and 308. The coolant absorbs heat from the melted resin 318 via the sides 304 and 308. Once cooling is completed, the liquid resin 318 becomes solid resin 318′, as shown in FIG. 11B. If thermosetting resin is used, then the liquid resin 318 converts to solid resin 318′ without cooling, and generally by heating using a liquid or electrical heating source in the mold. This heat source may be used to control the temperature of the cavity 316 during the solidification reaction.
Referring again to FIG. 11B, once the resin 318′ solidifies, the mold is opened. In a typical molding apparatus, only one of the sides 304 and 308 moves. Here, the A side 304 is held in a fixed position while the B side 308 and the die structure 312, are pulled away. Typically the sides 304 and 308 are designed such that solid resin 318′ easily releases from the A side 304 while being retained in the B side 308, as shown. In high speed manufacturing, removal of the solid resin 318′ from the B side 308 by hand or by external mechanical apparatus is typically impractical. Rather, the solid resin 318′ is pushed from the B side 308 by ejector pins 320 as shown in FIG. 11C. The ejector pins 320 are held in position by the retainer plate 324. To actuate the ejector pins 320, the molding press forces the ejector plate 328 toward the B side 308. As the ejector plate 328 moves, the ejector pins 320 slide past the cavity surface 332 of the B side 308 to support the solid resin 318′. The ejector plate 328 and ejector pins 320 are designed to push the solid resin 318′ sufficiently out of the B side 308 for complete removal.
Referring now to FIG. 11D, once the solid resin 318′ has been removed from the molding apparatus 300, the ejector plate 328 and ejector pins 320 are retracted such that the leading surfaces 336 of the ejector pins 320 are aligned to the B side cavity surface. The A side 304 and B side 308 are then closed to prepare for the next molding cycle.
A typical molding cycle sequence includes the steps of (1) closing the mold, (2) injecting the mold, (3) holding the injected material under pressure, (4) cooling the molded part, (5) opening the mold, and (6) ejecting the part. The overall molding cycle time is the sum of the time required to complete all of these steps. High volume manufacturing of resin-based articles requires minimizing the cycle time while producing consistent, high-quality parts. Typically, the times required to open, close, and eject the mold are short relative to the overall cycle time. Also, the mold close, open, and ejection times do not typically depend on specific mold cavity design or resin composition. However, the times required to completely inject the mold and to sufficiently cool the resin do depend on the mold cavity design and the resin composition.
Minimization of the injection and cooling times is essential for successful commercial molding. However, minimization creates a risk of scrap due to any of a variety of molding problems. For example, if liquid resin is injected too rapidly, then the molded part may exhibit burn marks, jetting, or excessive flash. Conversely, if injection is too slow, then molded part may exhibit flow marks, incomplete cavity fill (short shot), or splaying. Similarly, improper cooling may cause blistering, sink marks, or warping. An improper melt-front flow, due to poor mold design, may cause knit lines in the molded article. Efforts to minimize cooling time may worsen this problem.
Interactions between mold design, resin composition, and molding parameters—such as temperature, injection pressures, injection rates, cooling rates, screw speeds, and timing—are complex. Therefore, the plastics molder must carefully study the performance of the molding apparatus to optimize set-up to produce high-quality molded parts at optimal cycle times. Ideally, the molder would monitor the liquid resin 318 as it is injected into and flows through the mold cavity 316. However, the mold cavity 316 is buried inside an opaque block of metal. Therefore, the flow performance of the liquid resin 318 cannot be observed visually. Resin temperature and pressure are not directly accessible. Without direct observation or parametric data, the manufacturer must optimize performance by analyzing molded articles. This approach is time-consuming and ineffective.
One technique to provide molding data is to place temperature or pressure sensors directly inside the molding cavity. However, there are several problems with this approach. The presence of the sensors in the cavity must be planned in advance or extensive and costly modifications of the mold must be done after the mold is built. Frequently, the manufacturer molds plastic components for others and does not own the mold die. Further, he may be contractually prohibited from altering the molding cavity in any way. In such cases, the placement of sensors within the mold cavity, after the initial mold build, is simply not an option. A significant need exists for a method to acquire, real-time mold cavity parametric data without altering the mold components cavity.